Many shows have characters or their traits that serve as the show's recurring little jokes - characters that are talked about but never appear on screen, characters that never talk, etc.

On Cheers, Norm (the same guy Morn was called after) talked about his wife Vera in every episode, but she was never seen.

On Frasier, Niles and other characters often talk about Niles' wife Maris, she was even an important character in some storylines, and we actually found out a lot about her personality, habits etc. - but she never appeared on screen.

On My So-Called Life, characters often (though not in every episode) talked about a guy called Tino, but he never appeared on screen.

On The Simpsons, Maggie (an eternal baby, since the characters on that show never age) normally never speaks a word; the exception is an episode called "Maggie's first word", where Maggie speaks only at the end of the episode, when nobody can hear her, and says just one (important) word (they actually had Liz Taylor guest star just to lend her the voice!). Other exceptions happen only in Halloween episodes, where Maggie spoke 2 times (to my knowledge), both times in completely freakish situations and in a deep masculine voice. There was also an episode set in the future, in which we see an adult Maggie who's become a famous singer, and everyone keeps talking about her great voice - but again we don't hear her.

Deep Space Nine had such a recurring joke with Morn never speaking a word, while everyone describes him as extremely talkative.